1. Field of the Invention
The present invention relates to a battery status detecting method and a battery status detecting apparatus for detecting the status of a secondary battery that supplies power to an electrical load. The present invention also relates to a method of deriving an expression for calculating the dischargeable capacity of a secondary battery that supplies power to an electrical load.
2. Description of the Related Art
Open circuit voltage characteristics of a secondary battery indicating a relationship between the charge rate and the open circuit voltage of the secondary battery are conventionally assumed to be substantially constant regardless of degradation or changes in the usage conditions of the secondary battery. For example, Japanese Laid-Open Patent Publication No. 2001-231179 discloses a battery capacity detecting method using such open circuit voltage characteristics. Specifically, the disclosed battery capacity detecting method includes measuring the open circuit voltage during a rest period of a secondary battery and estimating the charge rate of the secondary battery at the time of the measurement based on the measured open circuit voltage and the open circuit voltage characteristics of the secondary battery obtained beforehand. It is noted that the above publication further discloses a method of estimating the full charge capacity of the secondary battery based on the charge rates of the secondary battery before and after performing battery charging operations and the amount of power charged to the secondary battery during the battery charging operations, and a method of estimating the remaining capacity of the secondary battery after completion of battery charging operations based on the charge rate after performing battery charging operations and the full charge capacity.
It is noted that various methods have been developed for estimating the remaining capacity of a secondary battery including the above-described method. However, operations of an electrical load receiving power supply from a secondary battery may be restricted by the voltage (power supply voltage) of the secondary battery rather than the remaining capacity of the secondary battery, and in this case, even when the secondary battery has adequate remaining capacity, an operation failure may still occur if the voltage of the secondary battery is below a required voltage for enabling operations of the electrical load. Because conventional methods do not take into consideration such characteristics of the electrical load that receives power from the secondary battery, the above-described risk of operation failure still exists even when the remaining capacity of the secondary battery is estimated.